1. Field of the Invention
This invention relates generally to pressure sensing switch arrays and more specifically concerns switch arrays for the detection of the presence or absence of a person from a hospital bed, wheel chair, medical seating structure, baby carriage or any other body supporting structure to which it may be useful to determine the status of occupancy, and to the method of making such switches.
2. Description of the Related Art
Pressure sensing switch arrays presently used in many medical facilities are disclosed in previously issued U.S. Pat. Nos. 4,484,043 and 4,565,910. In these known devices, broad bands of conductive material are used in upper and lower layers in such a way that contact between any portion of the upper and lower layer completes an electrical circuit. While this type of switch array works reasonably electrically, they are somewhat difficult and slow to manufacture in a manner providing consistency and predictability of operation and consequently sufficiently expensive to limit their broad use as a single use disposable device. Equivalently they have difficulty in reliably sensing patient body mass weights of less than 100 pounds. These known devices are also of relatively thick profile and stiffness to provide adequate separation of the conductive bands thereby resulting in diminished patient comfort when in use. Also, these known devices are not completely sealed and therefore do not provide a fluid impervious device, potentially compromising the devices integrity and consistency of operation.
An alternative pressure sensing switch design is disclosed in previously issued U.S. Pat. Nos. 5,554,835 and 5,623,760. The functionality of the device identified in these known patents derives from membrane switch technology comprising an upper, middle and lower laminar elongated members. Electrically conductive elements affixed to the lower surface of the upper member traverse cavities provided by the structure of the middle member. A second array of electrically conductive bands are fixed to the upper surface of the lower member and equivalently traverse the cavities provided by the middle member and the upper member bands. Selected lower member conductive bands are discretely connected to pins of an electrical input connector and the other lower member bands are discretely connected to alternative pins of the same electrical connector forming an output lead. An array of substantially parallel spaced apart dielectric bands is fixed to the lower member upper surface and traverses the cavities between the first and second arrays of conductive bands at a forty-five degree angle in such a way to separate the first and second arrays of conductive bands from making electrical contact with each other in the area of overlap between the dielectric bands.
Pressure derived electrical activation of the first described pressure sensitive switch design identified in U.S. Pat. Nos. 4,484,043 and 4,565,910 is more frequently achieved by a crimping or kinking of the device than by direct pressure application through the exterior of the upper or lower surface of the device. Pressure derived electrical switch activation of the second described device disclosed in U.S. Pat. Nos. 5,554,835 and 5,623,760 is achieved by direct pressure application to the outer surface of either the upper or lower member or by flexation of the device. It is, however, a function of the second described device that reliable pressure derived switch activation typically occurs only when direct pressure is applied to a small surface area location--typically less than one square inch--at any given location on the outer surfaces of the upper or lower members. Equivalent, or even substantially greater pressure when applied to a larger surface area--up to and including the entire surface area of the device--will not reliably result in electrical switch contact within the device unless the device exhibits considerable flex under such pressure or the applied direct pressure over a large surface area of the device exceeds one hundred pounds body mass weight or greater. Additionally, this described device exhibits complexity and expense in manufacturing and operation due to its utilization of an electronic style electrical connector to permit the inflow and output of a suitable derived external driver current to be applied to the device. The pressure sensing switch array of the present invention has specific design features which distinguish it from the prior art devices.